Question

Which of the following is the correct order of events in mitosis? a. Sister chromatids line up at the metaphase plate. The kinetochore becomes attached to the mitotic spindle. The nucleus re-forms and the cell divides. The sister chromatids separate. b. The kinetochore becomes attached to the mitotic spindle. The sister chromatids separate. Sister chromatids line up at the metaphase plate. The nucleus re-forms and the cell divides. c. The kinetochore becomes attached to metaphase plate. Sister chromatids line up at the metaphase plate. The kinetochore breaks down and the sister chromatids separate. The nucleus re-forms and the cell divides. d. The kinetochore becomes attached to the mitotic spindle. Sister chromatids line up at the metaphase plate. The kinetochore breaks apart and the sister chromatids separate. The nucleus re-forms and the cell divides.

Short answer

Answer: The correct order of events in mitosis is the kinetochore becoming attached to the mitotic spindle, sister chromatids lining up at the metaphase plate, sister chromatids separating, and finally, the nucleus re-forming and the cell dividing.

Step-by-step solution

Kinetochore attaches to mitotic spindle

During mitosis, the kinetochore of each chromosome becomes attached to the mitotic spindle - a structure composed of microtubules. We can eliminate option (c) because it states the kinetochore becomes attached to the metaphase plate, which is incorrect. The other three options have the kinetochore correctly attached to the mitotic spindle.

Sister chromatids line up at the metaphase plate

Once the kinetochore is attached to the mitotic spindle, the sister chromatids line up at the metaphase plate. This process is called metaphase. Option (b) incorrectly states that sister chromatids separate before lining up at the metaphase plate, so we can eliminate it.

Sister chromatids separate

After the sister chromatids are lined up at the metaphase plate, they separate from each other through a process called anaphase. Both options (a) and (d) have this step in the correct order.

Nucleus re-forms and cell divides

Lastly, the nucleus re-forms, and the cell divides in a process called cytokinesis. Both options (a) and (d) correctly mention this step.

Conclusion

Comparing the remaining options, we can see that option (a) has the sister chromatids separating last, while option (d) has the kinetochore breaking apart and the sister chromatids separating before the nucleus re-forms and the cell divides. The correct order of events is present in option (d).

Key concepts

Kinetochore

The kinetochore is a critical protein structure on chromosomes where the spindle fibers attach during cell division. Its main role is to ensure the chromosomes segregate correctly. This attachment is crucial as it safeguards the equal distribution of genetic material into the daughter cells. It interacts with the microtubules of the mitotic spindle at specific points, helping guide chromosomes to their proper location. Think of it as the clasp holding everything together during mitosis. Without a properly functioning kinetochore, the risk of unequal distribution of chromosomes, known as aneuploidy, increases.

Metaphase plate

During mitosis, the cell reaches the metaphase stage, characterized by the alignment of chromosomes along the metaphase plate. The metaphase plate itself is not a physical structure but rather an imaginary line or plane in the middle of the cell. This alignment ensures that when separation occurs, each new cell receives an identical set of chromosomes. Chromosomes line up due to the tension created by the spindle fibers as they pull on the kinetochores. This precise alignment is essential for genetic stability and the prevention of errors during cell division.

Anaphase

Anaphase is the phase in mitosis where sister chromatids separate and are pulled toward opposite poles of the cell. This movement is facilitated by the shortening of microtubules attached to the chromatids.

• The separation ensures that each daughter cell ends up with the same genetic material.
• The kinetochore plays a significant role in this process by holding the chromatids until the correct moment for separation.

The transition into anaphase is marked by a sudden and synchronized movement of chromatids. Any irregularities during this stage can result in genetic anomalies being passed on to daughter cells.

Cytokinesis

Cytokinesis is the final step of cell division, where the cell physically divides into two new daughter cells. It occurs after the mitotic phase, finalizing mitosis. During cytokinesis, the cell's cytoplasm is divided, forming distinct and separate cells.

• In animal cells, this division is achieved through a contractile ring that pinches the cell into two.
• In plant cells, cytokinesis is accomplished by constructing a cell plate in the center, which eventually develops into the cell membrane and cell wall.

This final step ensures that both cells are fully equipped to become independent and functional on their own, each with its own complete set of identical chromosomes.